An electrode for a crystal solar cell is required to have low electrical resistance to facilitate improved efficiency. In the case of a contact between a metal and semiconductor, in such structures, a Schottky barrier is known to be formed that causes a considerable increase in contact resistance. Since the electrical resistance of an electrode is the sum of the conductor resistance and the contact resistance, in addition to lowering the conductor resistance of an electrode, it is also necessary to reduce the contact resistance with the conductor.
As shown in FIG. 1, a conventional silicon solar cell is a, so called, P-type base solar cell 105 which has a n-type diffusion layer (n+ layer) 102 on a p-type silicon substrate 101, an anti-reflection coating and a n-type electrode 104 on the p-type diffusion layer. The n-type electrode 104 is formed in line and called bus or finger electrode. In the backside, a p-type electrode consists of aluminum layer 106 and silver layer 107.
FIG. 2 shows an N-type base solar cell 205 that has been on tested for practical use. The N-type base solar cell 205 has a p+ layer 202 on a p-type silicon substrate 201, an anti-reflection coating 203 and a p-type electrode 204 on the p+ layer 202. In the backside, an n-type electrode 206 is formed.
J. E. Cotter et al. says that n-type silicon wafers are more tolerant to chemical and crystallographic defects than p-type silicon wafers (P-type versus n-type Silicon Wafers: Prospects for hi-efficiency commercial silicon solar cell; IEEE transactions on electron devices, VOL. 53, No. 8, august 2006). L. J. Geerligs, et al. says that high and homogeneous carrier lifetime was observed in the n-type silicon wafer (N-TYPE SOLAR GRADE SILICON FOR EFFICIENT P+N SOLAR CELLS: OVERVIEW AND MAIN RESULTS OF THE EC NESSI PROJECT; European photovoltaic solar energy conference and exhibition 4-8 Sep. 2006).
Aluminum paste which has both a relatively low conductor resistance and contact resistance has been used for forming a p+ electrode in both N-type and P-type base solar cells. However the lower conductor resistance and contact resistance are now required. The problem is that Silver, which has lower conductor resistance, is hardly ever used because it gets high contact resistance on the contact with p+ layer. To solve the problem, JP2006-93433 discloses an electrode which is formed on a p-type conductive layer by using a conductive paste which comprises silver powder, an organic vehicle, glass frit and at least one composition selected from the group consisting of boron powder, an inorganic boron compound, and an organic boron compound.
It is desirable to provide a p-type electrode which has lowered contact resistance as well as lowered conductor resistance.